Medical implants are often delivered to anatomical sites within a patient's body to treat focal defects. For example, damaged cartilage or bone within a patient's body may include a cavity or void formed by trauma, disease, or surgery. The cavity or void may leave the bone prone to further injury or damage, especially when the cavity or void is formed in a weight-bearing joint such as the knee. To treat such a defect, the affected tissue may be drilled out or otherwise prepared to receive an implant designed to promote tissue formation. The implant may comprise healthy cartilage and/or bone cut from other locations on the patient's body, tissue(s) harvested from a donor, and/or synthetic material(s) such as porous ceramics or metals, biocompatible polymers, or combinations thereof.
Preparing a defect to receive an implant can be a challenging task. Ideally, the defect should be prepared to a size and shape corresponding to that of the implant. This includes not only the dimensions of the prepared cavity, but also its orientation relative to the surrounding tissue surface. For example, bone plugs and similar implants are often cut from bone at angles substantially perpendicular to the surface of the bone. If such a bone plug is delivered into a cavity drilled at an angle to the surrounding surface, the end of the bone plug may not properly correspond to the surrounding surface. Portions of the bone plug may extend out of the cavity (i.e., be “proud”) so as to create an undesirable protrusion relative to the surrounding surface. Alternatively or additionally, portions of the bone plug may be recessed relative to the surrounding surface such that a defect in the bone remains.
Conventional techniques for properly orienting an instrument, such as a cutting tool, relative to a tissue surface rely solely upon visual and/or tactile feedback. After placing the cutting tool in contact with the tissue surface, a surgeon may simply change the angle of the cutting tool until she believes it looks or feels perpendicular to the tissue surface. The surgeon then attempts to maintain this orientation throughout the procedure. As can be appreciated, such techniques involve a great degree of approximation and may still result in cavities with undesirable orientations. Moreover, oftentimes the surgeon cannot directly view the tissue surface (e.g., because the procedure is performed arthroscopically). Attempting to properly orient a cutting tool while looking through an arthroscope can be even more challenging due to the magnification of the arthroscope.
As a result, various devices have been developed to facilitate preparing a defect to receive an implant. For example, U.S. Pat. No. 5,885,293 to McDevitt discloses a tool for cutting bone surfaces at perpendicular angles. The tool includes a cylindrical bone cutter, a cylindrical housing mounted on a handle of the bone cutter, a probe that slides within an inner bore of the bone cutter and the housing, and a ring on the exterior of the housing connected to the probe by a pin extending through the housing. The probe is biased to normally extend beyond the end of the bone cutter. When the bone cutter is placed against a bone surface, the probe is displaced into the inner bore. This displacement causes the ring to slide along the exterior of the housing, which includes indicia so that the displacement of the probe may be viewed. A surgeon manipulates the tool until maximum displacement is achieved, which, according to McDevitt, which generally corresponds to perpendicular alignment.
Although McDevitt offers an alternative to conventional techniques for orienting a tool relative to a bone surface, there remains room for improvement. The maximum displacement of a single probe may not always provide a reliable indication of perpendicular alignment. This is particularly true when using the tool disclosed in McDevitt to determine the proper orientation for repairing a defect on a tissue surface. For example, because the probe is a cylindrical element that slides along the center axis of the bone cutter, the probe is likely to contact the cavity defining the defect when the bone cutter is brought into contact with the bone surface. The maximum displacement of the probe found after manipulating the bone cutter may not necessarily reflect perpendicular alignment due to the differences (e.g. irregularities) between the defect and the profile of the surrounding surface.
Accordingly, an improved instrument to determine the proper orientation for repairing a defect on a tissue surface would be highly desirable.